This invention relates generally to multi-phase transformer systems and more particularly to multi-phase power transformer systems having improved phasor balance and reduced total harmonic distortion (THD).
As is known in the art, many electrical systems require direct current power. Such direct current (DC) is typically produced by rectifying three-phase alternating current (AC) voltage. The rectifiers, however, induce harmonic distortion in the input line. Such effect is described in U.S. Pat. No. 4,779,181 entitled xe2x80x9cMultiphase Low Harmonic Distortion Transformerxe2x80x9d, inventors Traver et al., issued Oct. 18, 1988. The total harmonic distortion (THD) generated by rectification can be improved by increasing the number of AC phases fed to the rectifiers. Some of these multi-phase transformer systems are described in the U.S. Pat. Nos.: 4,779,181, 4,255,784, 5,148,357, 4,532,581, and 4,488,211. The line harmonics for these systems are inversely proportional to the number of phases according to the following equation:
KH=2*m*(n+/xe2x88x921),
where KH is the harmonic order
m is the number of phases
n =0, 1,2, . . .
For example, the harmonics of a 12-phase system are: 23, 25, 47, 49, 71, 73.
A schematic diagram for a conventional 18-phase, single transformer is shown in FIGS. 1A and 1B. Thus, the transformer 10 has a three-phase primary winding 12 magnetically coupled to a secondary winding section 16 through a core 14. The secondary winding section 16 has a set of six main Y-configured, three-phase secondary windings 16a-16f. The voltage produced in the three secondary windings of set 16a are {overscore (A)}, {overscore (B)} and {overscore (C)} where {overscore (A)}, {overscore (B)} and {overscore (C)} have equal magnitudes and 120 degrees of relative phase shift with respect to each other. The voltages produced in the three secondary windings of set 16b are K1 {overscore (A)}, K1{overscore (B)} and K1{overscore (C)} where K1 is a less than one. Thus, the number of turns in each of the three windings in set 16a are equal to each other and the number of turns set 16b are equal to each other the number of turns in the three sets of windings in set 16b are a fraction of the number of turns in the three windings in set 16a. Thus, the voltages K1{overscore (A)}, K1{overscore (B)} and K1{overscore (C)} have equal magnitudes, here 1/K1 th the voltage in each of the windings set 16, and 120 degrees of relative phase shift with respect to each other. That is, the voltages K1{overscore (A)}, K1{overscore (B)} and K1{overscore (C)} are in-phase with the voltages {overscore (A)}, {overscore (B)}, and {overscore (C)} in set 16a. In like manner, the voltages in sets 16c through 16f are: K2{overscore (A)}, K2{overscore (B)} and K2{overscore (C)}; K3{overscore (A)}, K3{overscore (B)} and K3{overscore (C)}; K4{overscore (A)}, K4{overscore (B)} and K4{overscore (C)}; and K5{overscore (A)}, K5{overscore (B)} and K5{overscore (C)}, respectively, where K3=1, K1=K4, K2=K5, K2 less than K1 and such relationship is determined by the relative number of turns in the windings.
The secondary section 16 also includes six sets 16g-16l of auxiliary windings magnetically coupled to the primary 12 though core 14. Each set has three windings. Each one of the windings in the set produces a voltage in-phase with a corresponding one of the three voltage {overscore (A)}, {overscore (B)}, and {overscore (C)}. The magnitudes of the voltages in sets 16g-16l are scaled relatively to the magnitudes of the voltages {overscore (A)}, {overscore (B)}, and {overscore (C)} by factors of: 1/K6 through 1/K11, respectively. It is noted that the windings in sets 16a through 16f are connected to the windings in sets 16g through 16l selectively as shown to thereby produce voltages VO1 through VO18 which may be represented as: {overscore (C)}+K6{overscore (A)};
{overscore (A)}+K6{overscore (B)};
{overscore (B)}+K6{overscore (C)};
K1{overscore (C)}+K7{overscore (A)};
K1{overscore (A)}+K7{overscore (B)};
K1{overscore (B)}+K7{overscore (C)};
K2{overscore (C)}+K8{overscore (A)};
K2{overscore (A)}+K8{overscore (B)};
K2{overscore (B)}+K8{overscore (C)};
K3{overscore (C)}+K9{overscore (A)};
K3{overscore (A)}+K9{overscore (B)};
K3{overscore (B)}+K9{overscore (C)};
K4{overscore (C)}+K10{overscore (A)};
K4{overscore (A)}+K10{overscore (B)};
K4{overscore (B)}+K10{overscore (C)};
K5{overscore (C)}+K11{overscore (A)};
K5{overscore (A)}+K11{overscore (B)};
K5{overscore (B)}+K11{overscore (C)}, respectively.
These voltages VO1 through VO11 are fed to a rectification system, as shown. The rectified voltages are combined in combiner 20 to produce the here 18-phase combined output voltage, VOUT.
In accordance with the present invention, a multi-phase transformer system is provided having a main transformer fed by an N-phase voltage and a separate auxiliary transformer fed by the N-phase voltage. Windings in the main transformer are connected to secondary windings in the auxiliary transformer to provide pairs of connected windings. Each pair of connected windings has one of the windings of the main transformer and one of the secondary windings of the auxiliary transformer. The windings in such connected pair are arranged to produce voltages having different phases with each pair of windings producing an output voltage equal to the vector sum of the voltages produced by the such connected pair of windings.
With such an arrangement, by having two separate transformers, i.e., the main transformer and the auxiliary transformer, fabrication of a multi-phase transformer system is simplified. Further, leakage inductance in the auxiliary transformer may be readily adjusted and increased to thereby suppress higher harmonic distortion without the need for an additional filter. The increased leakage inductance of the auxiliary transformer does not cause higher harmonic distortion in the low frequency part of the spectrum that occurs if the leakage inductance of the main transformer is increased.
In one embodiment, secondary windings of the main transformer are connected to the secondary windings of the auxiliary transformer. In such embodiment, each pair of connected windings includes one of the secondary windings of the main transformer and one of the secondary windings of the auxiliary transformer.
In a second embodiment, the N-phase voltage is connected directly to the primary winding of the auxiliary transformer and indirectly to the primary windings of the main transformer through the secondary windings of the auxiliary transformer. In such embodiment, each pair of windings includes one of the secondary windings of the auxiliary transformer and one of the primary windings of the main transformer.
In accordance with still another aspect of the invention, a multi-phase transformer system is provided having a main transformer and a separate auxiliary transformer. The main transformer includes a main secondary winding section magnetically coupled to a main primary winding section. One of the winding sections of the main transformer includes a plurality of M sets of main windings, where M is an integer greater than one. Each one of the M sets has a plurality of N main windings for producing N voltages having the same amplitudes and a predetermined phase relationship. The amplitudes of the voltages produced by one of the sets are different from the amplitude of the voltages produced by another one of the sets. The auxiliary transformer includes an auxiliary primary winding section having inputs connected to the main transformer. The auxiliary transformer includes a plurality of M auxiliary secondary winding sets magnetically coupled to an auxiliary primary winding section. Each one of the M sets of auxiliary secondary winding sets is connected to a corresponding one of the M sets of main windings. Each one of the auxiliary secondary windings in each one of the M sets thereof produces N voltages having the predetermined phase relationship. The amplitudes of the voltage produced in each one of the M sets of auxiliary secondary windings are equal. The amplitudes of the voltages produced in one of the M sets of auxiliary secondary windings are different from the amplitudes of the voltages produced in another one of the sets M sets of auxiliary secondary windings. Each one the windings in each one of the sets M sets of auxiliary secondary windings is connected to a corresponding one of the windings in the one of the M sets of main windings to form a pair of connected windings. The windings in the connected pair produce voltages having different amplitudes and phases. Each one of the connected pair of windings produces an output voltage equal to the vector sum of the voltages produced by the connected pair of windings.
In one embodiment, the M sets of main windings are secondary windings of the main transformer. In such embodiment, each pair of connected windings includes one of the secondary windings of the main transformer and one of the secondary windings of the auxiliary transformer.
In a second embodiment, the M sets of main windings are primary windings of the main transformer. In such embodiment, each pair of windings includes one of the secondary winding of the auxiliary transformer and one of the primary windings of the main transformer.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.